THE OZONE MOLECULE
Ozone is an allotrope of oxygen and has the chemical formula O3. It is less stable than the common structure of oxygen (diatomic, O2), decomposes to the diatomic form of oxygen. It is also a very strong form of oxidising agent. The molecular shape is bent, as predicted by the VSEPR (Valence Shell Electron Pair Repulsion) Theory, with a bond angle of around 117 degrees. The resonance hybrid contains each O-O bond with bond order of 1.5, by 2 resonance contributing structures.1
Figure 2: A Lewis Structure showing the resonance hybrid of O3
THE OZONE LAYER
The earth’s atmosphere can be divided into five layers, from highest to lowest. This is shown in Figure 3 (description below) on the right.
Exosphere: 700 to 10,000 km
Thermosphere: 80 to 700 km
Mesosphere: 50 to 80 km
Stratosphere: 12 to 50 km
Troposphere: 0 to 12 km
The stratosphere is where the ozone layer is located. Ozone concentration is around 10ppm in the ozone layer, which is much higher at the earth’s surface (0.3ppm).2
THE CHAPMAN CYCLE
The ozone-oxygen cycle, also known as the chapman cycle, is the process which ozone is regenerated in the stratosphere.
Creation
Oxygen molecule is split into two oxygen radicals, by high frequency UV light.
Oxygen radicals then combine with an oxygen molecule to form an ozone molecule.
O2 + ℎν → 2 O•
O• + O2 → O3
Chapman Cycle
Ozone molecule absorb radiation (UV-B and UV-C) and decomposes into oxygen molecules and oxygen radicals.
The oxygen radicals then react with oxygen molecules again to form back Ozone, this process is exothermic and releases kinetic energy.
O3 + ℎν (240–310 nm) → O2 + O
O + O2 → O3 + EK
Removal
When two oxygen radicals react, then can form back an oxygen molecule.
An oxygen radical can also react with an ozone molecule to form 2 oxygen molecules.
O3 + O· → 2 O2
2 O· → O2
Hence, the net reaction is 2 O3 → 3 O2
References
1 http://www.ozonelayer.noaa.gov/index.htm
2https://www.nasa.gov/mission_pages/sunearth/science/mos-upper-atmosphere.html#.VHg5AzHF8vY
Figure 2: http://chem-net.blogspot.sg/2012/01/simple-procedure-for-writing-lewis.html
Figure 3: A Diagram showing the Layers of the Earth’s Atmosphere. http://ete.cet.edu/gcc/?/volcanoes_layers/